The high level of automation used in fabricating semiconductor devices relies on sophisticated handling and transport equipment for moving semiconductor wafers between various processing stations. Most handling and transport operations are conducted under automatic control using a programmable logic controller, or other programmed computer which issues control signals for operating the equipment with little or no intervention by an operator. Nevertheless, there are certain situations where operator intervention becomes necessary, consequently the automated wafer handling equipment mentioned above normally includes a series of manual controls that permit the operator to separately control each stage of movement of the wafers. By way of example, standard mechanical interface (SMIF) pods are used to transport a number of wafers which are typically stored in cassettes. The SMIF pod normally includes robotically operated arms that transport wafer cassettes to a stage from which the cassette is in turn automatically transported to another area, or into processing equipment, such as a deposition chamber.
The transport of the wafer-carrying cassette just described is normally carried out completely automatically, without operator intervention. However, the control system for the handling equipment normally has an off-line mode of operation that can be used by an operator to individually control movements of the transport, which might be required during the initial set up of the equipment in preparation for processing a batch of wafers, or to either clean the equipment or test it to ensure that the movements are smooth and accurate. Because the SMIF pod, the transport arms and the stage are all capable of moving in various directions, the possibility exists that a collision may occur between these elements if the operator does not properly operate the manual controls, and in the proper sequence of control movements. For example, the SMIF arm could be actuated so as to attempt to move a cassette onto a stage before a previously placed cassette could be removed from the stage, thus resulting in a collision that could easily damage or destroy wafers and/or the handling equipment. Up to the present time, there have been no means provided for avoiding such collisions of equipment and resultant damage to wafers. Consequently, many wafers and transport equipment are damaged in this manner, causing undesirable down time as well as reduced manufacturing throughput.
It may thus be appreciated that there is a clear need in the art for a safety interlock system, which overcomes each of the deficiencies described above, and prevents damage to wafers and handling equipment when the equipment is under manual control by an operator.